When proteins or blood platelets are deposited on a medial separation membrane in contact with body fluid or blood, they can cause a reduction in the performance of the separation membrane or a biological reaction, which raises a serious problem. Also when a water treatment membrane is used in a water purifier or the like, the deposition of proteins or organic substances causes a reduction in the performance of the separation membrane. In attempts to solve these problems, various studies have been made by hydrophilizing separation membranes. For example, there are disclosed methods that include mixing polyvinylpyrrolidone, a hydrophilic polymer, with polysulfone in the step of preparing a membrane forming stock solution and then subjecting the stock solution to a membrane-forming process so that a membrane having hydrophilicity and prevented from being fouled can be produced (Patent Document 1). However, these methods have certain limitations such as the need for a large amount of a hydrophilic polymer in the membrane forming stock solution for imparting hydrophilicity to the surface, the need to limit the hydrophilic polymer to one with compatibility with the base polymer, and the need to examine the optimal stock solution composition depending on the intended use of the material.
Patent Document 2 discloses a method of hydrophilizing a membrane by coating the membrane with polyvinyl acetal diethylamino acetate and a hydrophilizing agent. In this method, the hydrophilizing agent is covered with polyvinyl acetal diethylamino acetate, so that the deposition resistance effect may be drastically reduced. In addition, when the membrane is immersed in each of a polyvinyl acetal diethylamino acetate solution and a hydrophilizing solution, the separation performance of the membrane may be reduced.
There are also disclosed a method that includes making a hydrophilic component such as polyvinylpyrrolidone water-insoluble by radiation or heat so that the hydrophilic component can be introduced into a membrane being produced (Patent Document 3) and a method that includes bringing a polysulfone-based separation membrane into contact with a solution of a hydrophilic polymer such as polyvinylpyrrolidone and then forming an insolubilized coating layer by radiation crosslinking (Patent Document 4). However, there is a problem in which the intermolecular interaction between the aqueous polymer such as polyvinylpyrrolidone and the polysulfone-based polymer is weak, so that the coating layer is difficult to form.
Thus, there is disclosed a method that includes bringing an aqueous solution of a polyvinyl alcohol with a saponification degree in a certain range into contact with a polysulfone-based separation membrane so that a coating layer can be efficiently formed on the membrane surface by hydrophobic interaction between polysulfone and vinyl acetate (Patent Document 5). As a result of studies by the inventors, it has been found that when a separation membrane is simply coated with polyvinyl alcohol according to the publication, the performance of the separation membrane is significantly reduced, because the method disclosed in the publication does not relate to the deposition resistance. It is also known that the hydroxyl group of polyvinyl alcohol tends to activate complements, when brought into contact with blood.
It is also said that even when a material surface is coated with a hydrophilic polymer such as polyvinylpyrrolidone or polyethylene glycol, the deposition of proteins and so on can be only temporarily inhibited (Non-Patent Document 1). Under the circumstances, a separation membrane module having a high-performance membrane and satisfactory compatibility with blood has not yet been established.    Patent Document 1: Japanese Patent Application Publication (JP-B) No. 02-18695    Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No. 08-131791    Patent Document 3: JP-B No. 08-9668    Patent Document 4: JP-A No. 06-238139    Patent Document 5: JP-A No. 2006-198611    Non-Patent Document 1: Iryo Nanotechnology (Medical Nanotechnology), Kyorin-Tosho, pp. 115-116